1. Field of the Invention
The present invention relates to a rotary actuator assembly in a magnetic disk drive unit or an optical disk drive unit.
2. Description of the Related Art
In recent years, a magnetic disk drive unit as a kind of external storage for a computer has been desired with reduced size and enlarged capacity. In association with a reduction in size of the magnetic disk drive unit, it has been demanded to manufacture a compact, high-performance, low-cost magnetic circuit to be used in a voice coil motor (VCM) for driving an actuator assembly to be mounted in the magnetic disk drive unit. Further, a carriage lock or an actuator lock is generally used to prevent damage to magnetic disks in transporting the magnetic disk drive unit, and a low-cost reliable actuator lock is desired.
The actuator lock operates to lock the actuator assembly near an innermost cylinder or an outermost cylinder of the magnetic disks. In recent years, the actuator lock employing a solenoid or the like has become a primary actuator lock. However, when a trouble such as malfunction of the solenoid occurs, the magnetic disks are possibly damaged to cause a reduction in reliability of the magnetic disk drive unit.
In the magnetic circuit used in the voice coil motor (VCM) for driving the actuator assembly in the magnetic disk drive unit, it is desirable to ensure a constant magnetic flux density in gaps in the range of swinging of a coil. In the conventional magnetic circuit, however, the nearer the coil is moved to both ends of the magnetic circuit, the more the gap magnetic flux density is reduced. Although this problem can be solved to some extent by extending an inner magnet and an outer magnet in their circumferential direction, the volume of each magnet is increased to cause an increase in cost and the gap magnetic flux density over the range of swinging of the coil is reduced because of yoke saturation. Further, it is known that a short coil is located in the gaps of the magnetic circuit to quicken the rising of VCM current at seeking acceleration.
The actuator lock mechanism employing the solenoid as mentioned above has a problem that the trouble such as malfunction of the solenoid causes damage to the magnetic disks, thus reducing the reliability of the magnetic disk drive unit. Further, there is another problem that the solenoid is relatively costly. It is desirable for the actuator assembly in the magnetic disk drive unit to suppress a reduction in gap magnetic flux density and obtain a fixed force constant irrespective of the position of the actuator assembly on any cylinder of the magnetic disks, so as to obtain a stable seeking performance and positioning performance.
According to the provision of the short coil in the gaps of the magnetic circuit for the purpose of quickening the rising of VCM current at seeking acceleration as mentioned above, it is necessary to widen the gaps by a space occupied by the short coil. As a result, the force constant of the VCM is generally reduced by the space. Compensation of such a reduction in the force constant requires enlargement of the magnetic circuit. However, since a form factor of the magnetic circuit is limited, it is difficult to realize the compensation of the reduction in the force constant.